Experience
Often, students work jobs that introduce them to their preferred career after school is over. This allows them to gain important introductory experience in the business and begin networking with those in that field. Even students who work in places not connected to their majors will look more impressive to potential employers. They'll have some kind of work experience on their resumes to start, and the fact that they've been able to hold a job while a school attests to their level of maturity, responsibility and time management. Employers look for employees who understand the work environment and work well as part of a team. Having a part-time job shows that the student is at least somewhat accustomed to the professional world, which will make the transition that much easier.